Someone posted that the heat pads use oxygen (hence come in seals packs) so a quick tip is to keep the foil pack, use the pad, then place it back in the foil pack and wrap some cling film around it. Works great for several reuses.
I believe this will work better than ziplock bags.
It's less about the money than it is putting something in the trash that still has 6+ hours of life.
Note: Take suitable fire precautions and store safely.
I've been getting two 3hr sessions out of each, using a ziplock bag and squeezing all the air out (by rolling) when I shut it.